HUNTINGTON, Utah -- Rescue officials at the site of last week's coal mine collapse in Utah say a fourth tunnel might have to be drilled to search for six missing miners.

Two holes have been sunk to an area where the men were believed to be working but no signs of life have been detected since the Aug. 6 collapse. A third tunnel was expected to reach the area Wednesday, said mine co-owner Robert Murray, president of Murray Energy Corp.

He said if the third tunnel yields no evidence of life, a fourth borehole is being considered to explore a region in the middle of the mining section, an area less likely to be life-sustaining, the Salt Lake Tribune reported.

"At that point, we're running out of possibilities," Murray told a news conference.

Crews have also been digging through the main entrance of the shaft and have cut through 750 feet, which is more than 1,700 feet from the collapse site, the report said.

Geologists have said mining activity caused the collapse while Murray claims an earthquake was responsible for the cave-in. He told reporters aftershocks undid much of the initial rescue work and forced crews to evacuate the mine.